An increase in particles high in the Earth’s atmosphere has offset some recent climate warming, the National Oceanic and Atmospheric Administration in Boulder said today.

“Since the year 2000, stratospheric aerosols (the particles) have caused a slower rate of climate warming than we would have seen without them,” said John Daniel, a physicist at the NOAA Earth System Research Laboratory in Boulder and co-author of the new study.

The study found that in the stratosphere, miles above the Earth’s surface, small, airborne particles reflect sunlight back into space, which leads to a cooling influence on the ground.

The study found a definite increase in stratospheric aerosol – or particles – since 2000.

“Stratospheric aerosol increased surprisingly rapidly in that time, almost doubling during the decade,” said Daniel. “The increase in aerosols since 2000 implies a cooling effect of about 0.1 watts per square meter – enough to offset some of the 0.28 watts per square meter warming effect from the carbon dioxide increase during the same period.”

Ryan Neely, of the University of Colorado and the Cooperative Institute for Research in Environmental Sciences, said the reason for the 10-year increase in the particles is not fully understood.

Neely, who is the co-author of the study, said the likely sources are smaller volcanic eruptions and/or human activity. Daniel and his colleagues used a climate model to determine how the changes in particles could affect global climate change.

They found that models miss an important cooling factor if they don’t account for the influence of stratospheric aerosol, or don’t include recent changes in the stratospheric aerosol levels.

The study concluded that future global temperatures will depend on stratospheric aerosol.

The warming from greenhouse gases and aerosols calculated for the coming decade can vary by a factor or two.

If stratospheric aerosol levels continue to rise, temperatures will not rise as quickly as they would otherwise, the study concluded. However, if the particles decrease, temperatures would increase faster.

“The ‘background’ stratospheric aerosols are more of a player then we expected,” said Daniel. “The last decade has shown us that it doesn’t take an extremely high volcanic eruption for these aerosols to be important to the climate.”

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